Vibration detectors in the form of filling level measurement devices make use of the effect of the oscillation frequency and the oscillation amplitude are dependent on the respective extent to which the oscillating element is covered, while the oscillating element is in free space and can oscillate without any damping, it experiences a frequency and amplitude change as soon as it is partially or completely immersed in the filling material. As a consequence, it is possible to clearly determine when the filling material from the container has reached the predetermined filling level, on the basis of a predetermined frequency change (normally, the frequency is measured). In general, filling level measurement devices are used primarily to provide protection against overfilling or for protection against pumps running on no load.
Furthermore, the damping of the oscillation of the oscillating element is also influenced by the respective density of the filling material. When covered to a constant extent, there is thus a functional relationship with the density of the filling material so that vibration detectors are very highly suitable for determining not only filling levels but also densities. In practice, in order to monitor and identify the filling level and/or the density of the filling material in the container, the oscillations of the membrane are recorded, and are converted into electrical received signals by means of at least one piezoelement. The received electrical signals are then evaluated by evaluation electronics. When determining a filling level, the evaluation electronics monitor the oscillation frequency and/or the oscillation amplitude of the oscillating element and signal the “sensor covered” or “sensor uncovered” state as soon as the measured values fall below or rise above a predetermined reference value. An appropriate message can be passed to the operator by visual and/or audible means. Alternatively or additionally, a switching process is initiated; by way of example, an inlet or outlet valve on the container is opened or closed.
EP 0 985 916 A1 discloses an extremely advantageous variant of a transmitting/receiving unit, via which, firstly, the membrane of the vibration detector is caused to oscillate and via which, secondly, the oscillations of the membrane are recorded and are converted into electrical signals. The transmitting electrode and the receiving electrode are each essentially semicircular in shape and are arranged on the same face of a piezoelectric element in the form of a disk; the piezoelectric element itself is homogeneously polarized and has a circular cross section. The transmitting electrode and receiving electrode are positioned symmetrically with respect to an axis lying in the plane of the membrane, with the axis corresponding to one diagonal of the circular surface of the piezoelectric element.
The piezoelectric element is fitted with a ground electrode approximately over the entire surface area on the face which is opposite that face of the piezoelectric element to which the electrode structure is fitted. If the membrane and the housing of the vibration detector are conductive, it is sufficient for the ground electrode to make electrical contact with the housing via the membrane. If, for safety reasons, there is no electrically conductive connection between the membrane or the housing and the piezoelectric element, then an insulating layer is arranged between the membrane and the piezoelectric element. In this case, the ground electrode is connected to ground via a connecting electrode which is arranged on that face of the piezoelectric element to which the electrode structure is fitted.